The Hubble time of ∼ 14 Gyr is comparable to the ages computed for
the oldest known stars in the universe. This rough equivalence is reassuring.
However, the age of the universe – that is, the time elapsed since its original
highly dense state – is not necessarily exactly equal to the Hubble time.
We know that gravity exists, and that galaxies contain matter. If gravity
working on matter is the only force at work on large scales, then the attractive
force of gravity will act to slow the expansion. In this case, the universe
was expanding more rapidly in the past than it is now, and the universe is
younger than H −1
0 . On the other hand, if the energy density of the universe
is dominated by a cosmological constant (an entity which we’ll examine in
more detail in Chapter 4), then the dominant gravitational force is repulsive,
and the universe may be older than H −1
0 .